A high pressure sodium vapor lamp generally comprises an arc or discharge tube mounted within a light-transmissive envelope provided with a base forming two electrical connectors or terminals for a circuit adapted to receive this base, the electrical connectors being internally connected to the arc tube. The latter contain elemental sodium and can be provided with a heating body which can be a coil surrounding this tube and connected electrically in parallel with the tube to the connectors or terminals formed by the base.
It is desirable, and one of the objects of the invention as detailed below, to be able to operate the high pressure sodium vapor lamp at feed voltages in excess of 100 V but without separate starters or igniters, neglecting the input or intrinsic starting inductivity as a source of the arc-firing voltage.
There are various techniques for firing a high pressuring sodium lamp without using separate igniters. For example, one such system is described in U.S. Pat. No. 3,721,846 (see also U.S. Pat. No. 3,721,845) in which a heating body in the form of a coil surrounds the arc tube and is electrically connected to the terminals so that the arc tube can be heated and at least part of the sodium provided therein, transformed into the vapor state so that the ignition voltage of the sodium lamp is reduced and the latter thereby rendered more readily startable.
This technique for starting a sodium lamp has the disadvantage that the heating wire, for ignition exclusively by the line voltage utilizing the heating effect, must be brought to a comparatively high temperature so that its useful life is drastically limited. Furthermore, the elements to which the heating wire coiled around the arc tube may be connected, must be designed in light of the high degree of thermal expansion, thereby increasing the assembly costs. It is also a disadvantage that the switch which is used to interrupt the heating occuring in these systems, generally utilizing a bimetallic element as the acting switch member and hereinafter referred to as a bimetal, operates in the vacuum of the envelope surrounding the discharge tube and, in the event that ignition of the arc tube does not occur at the appropriate time (e.g. because of a momentary failure of ignition conditions and contact failures of the support fixtures in the arc lamp or a return of the arc tube to a nonexcited or base state) the autoinductive shock which can be generated can be comparatively high (e.g. of the order of 8 to 10 kV) upon opening of the vacuum switch and sufficient to damage the lamp or the accessories with which it is used and can even result in fire.
It is also known to provide arc tubes which are ignited by the generation of a high autoinductive voltage. One such approach is found, for example, in Geramn patent DE-PS No. 1 589 162. The disadvantage of this system lies in the comparative unreliability of the ignition since, when the switch interruption coincides substantially with a zero passage of the sine wave of the supply voltage, no autoinductive voltage is generated by operation of the switch and ignition cannot occur.
Capacitative ignition systems are also known as is, for example, described in U.S. Pat. No. 4,037,129. A disadvantage of this system is that with a supply voltage of 220 V, the arc tube cannot be ignited in a sodium vapor lamp having the usual zenon filling gas. In such cases it is necessary to utilize a Penning mixture as the filling gas, thereby reducing the operating life of the lamp and having a detrimental effect on the light output thereof.